The PYTHONPATH doesn't have much to do with it. It just tells the Python interpreter, whichever interpreter runs, where to find extra modules you want to load.

But either way, changing the default version on your system is dangerous, as other commenters have observed. You stand the risk of breaking existing package that are built against Python2 and use /usr/bin/env python to find their interpreters.

In fact, that's the wrong way to phrase it. Changing the default version of Python will break your entire Ubuntu system and cause lots of programs to just not work. It's also completely unnecessary.